A additional support to insert the boundry cordinates manually for creating the flightplan. The conventional way is to place the points on the map but addition to add/edit the existing point by providing the lat and long from the list.

Have you tried the KML import app from the market? You can draw in Google Earth Pro and export the KML or if you have an ASCII file of the coordinates you can import them into Google Earth pro or QGIS. Final adjustments could then be made in DroneDeploy. Maybe importing the CSV would be a better option than entry for your request?

Still having a option to know where the point on the boundry is exactly located and slightly move them by changing their latlong is still the easiest of them. It could improve the way actual covering of the area.
Like in the scenarios of highways and railways the entire project can be devided into the segments. And thus by entering the values for such segment it will be possible to create each segment.